Brazing of thin metal structures



Feb- 27 1949- v I l.. w. sHU'r'rs E1- AL 2191631 I BRAZING OF THIN METALSTRUCTURES Filed sept. 28,1938

"ummm n vPatented Feb. 27, 1940 UNITED STATES BRAzING oF THIN METALSTRUCTURES Leroy W. .Shutts and Philip S. Rosenberg, Lockport, N. Y.,assignors to General Motors Corporation, Detroit, Mich., a corporationof Dela- Ware Application September 28,I 1938, Serial No. 232.084

1 Claim.

This invention relates to the manufacture of heat exchange units andmoreparticularly to a process for joining the elements `of a radiator coreunit.

The difficulties experienced with soldered extruded tube radiators inaircraft use because of high temperature and vibration have led toefforts to develop a practical method of brazing Aextruded tube cores.Such tubes have a l0 Wall thickness usually about .006 inch, which istoo thin to permit brazing the edges of the hexagonal end with a handtorch or open flame because of the lack of temperature control and thelikelihood of vexcessive heating. Brazing an extruded tube core sectionin a controlled hydrogen atmosphere furnace .is unsatisfactory becauseit is not feasible to prelocate the brazing alloy in a satisfactoryposition for proper flow. If the core is placed in an oven with thetubes m .vertically disposed, the brazing material around the tubes willow into the joints in the lower face by gravity, but with uniform heatthroughout the core the brazing alloy intended for the joints in theupper face will simply melt and not flow properly to the joints. The use'of two different alloys of different brazing temperatures for thejoints in the respective faces has been considered with the idea offirst brazing the lower melting point alloy on the bottom face and thenturning over the core and re-heating to a higher temperature for thebrazing operation in the other face, but the annealing action on thetubes because of the higher temperature is objectionable.

To concentrate the brazing temperature, it is A here proposed to dipsuccessively the opposite faces of the core in a molten salt bath heldat proper temperature to melt and flow the brazing alloy down into thejoints by gravity. ,Thus the brazing heat is applied only to thesubmerged parts and the upper ends of the tubes in the exposed core facedo not reach a temperature sufcient to melt the brazing alloy providedtherefor. The immediate application of heat to the exact surfaces to bebrazed and the short time interval required for flowing the brazingalloy into the joints reduces the likelihood of an annealing action onthe tube material and is an important feature of the present process.

5o A primary object of the invention is to provide a method for brazingtogether the ends of very thin wall tubes in the assembly of a radiatorcore. A further object is to enable the construction of a core insections, the sections u subsequently to be bonded together in propernumber to complete a core'of whatever size is desired. To this end thetubes of individual sections may be bound together with light sheetmetal frames and bonded thereto in the salt bath brazing operation sothat later the frames 5 of several sectionscan be torch brazed together,the frames being of, suiciently thick stock to permit torch brazing.

In the accompanying drawing:

Fig. 1 shows in elevation an extruded tube l0 core. Y

Fig. 2 is a fragmentary view, partly in section, of a number of tubesbounded by a frame and .with brazing foil applied to the tubes prior toimmersion in the heated molten salt. j l5 Fig. 3 illustrates the partssubmerged in a salt furnace.

Fig. 4 is a detailed viewshowing the union be. tween frames of two core.sections and Fig. 5 is a detailed view of another type of 20 coreadapted to the salt bath brazing process.

The core shown in Fig. 1 is of a well known type and consists of aseries of extruded copper tubes' I--I, each having its opposite endsexpanded for a short length andvof hexagonal section as at 2-2. When thetubes are brought together with the hexagonal end portions 2-2 in nestedrelation, the flat walls thereof are in abutting contact at both ends ofthe assembly and the intermediate portions of the tubes are 80 spacedapart. With the abutting tube surfaces in both faces properly joined andsealed, the

exchange of heat in the use of the core is between fluid iiowing throughthe hollow tubes and fluid flowing between or exteriorly of the I5tubes.

For joining and sealing the enlarged end portions 2-2 of the tubes in aradiator core, each tube has placed on it, prior to assembly, apredetermined amount of brazing material, which, 40 when the tubes areassembled, will lie adjacent to the joints formed by the enlargedhexagonal portions of the tubes. The application of the brazing materialmay be eiected in any suitable fashion, as for example by spraying or by5 wrapping a thin foil around each tube as is illustrated at 3 in Fig.2. If the complete core is to be made up of a number of sections, as isillustrated in the drawing, the hexagonal end portions in each face areembraced by a band or frame 4. This frame, incidentally, will need to beof corrugated outline longitudinally thereof to iit in nested relationwith the flat, angularly arranged, exposed walls of the outermost tubesin the assembly. Optionally, a layer of brazing -foil may be placed asat 5 between the band and the tube ends.

Regardless of Whether the frame is employed, the brazing operationconsists of immersing successively the opposite faces of thetubeassembly in the molten salt contained within a suitable crucible B asindicated in Fig. 3. Salt bath furnaces in general use for heat treatingwill be found suitable to the present purpose. The submersion of theparts in the molten salt has the important advantage of reducing oreliminating completely oxidation in the joint surfaces incident to thetemperature increase to which they are subject. Consequently, a strongerand better bond is had.

Since the tube ends in the opposite faces of the assembly are separatelysubmerged in the furnace, the molten salt will be maintained at propertemperature to heat the submerged portions and melt the adjacent fusiblematerial for its gravity flow down the vertical tube surfaces and intothe joints between the contacting flat walls. Likewise the framing band4 is united to the tube ends through the fusion and coherence of theintervening brazing material. By proper control of the temperature andspace of time required for the brazing operation, and because of theconcentration-of applied heat, no annealing action will be produced inthe tube and the brazing material at the uppermost ends of the tubeswill remain below its melting point. The successive submersions ofopposite faces of the core sections completes the assembly.

It will be understood that the brazing operation may be performed on afull Isized core but if, because of limitations in the type of equipmentor other reasons, it is preferable to form the core in sections, thenthe frames of the Several sections may be joined together, as forexample by mating the frames as shown in Fig. 4 and braze joining theabutting surfaces at 1 with the aid of a hand torch.

This brazing` process may be practiced on other types of thin wallstructures requiring concentrated heat application at spaced points, asan instance of which Fig. 5 is incorporated in the drawing and shows oneend of a core comprising a number of spaced tubes 8, fitted tocorresponding tubular projections 9 in a header plate Ill, there being asimilar header plate fitted to the opposite ends of the tubes. The tubes8 and projections 9 are illustrated as being of uniform diameterthroughout and telescoped together. Optionally the interfitting portionsmay be tapered for seating one within the other, but in any event thebrazing material can be applied to the tubes as at Il adjacent thejoints and caused to fiow to the joint surface upon immersion in theheated molten salt, first of one end face and then of the other, asbefore described.

We claim: v

In the manufacture of heat exchangers of the type having a number ofspaced tubes with opposite end portions expanded and nested together,the method of brazing said end portions comprising applying solidcoatings of brazing material to portions of the tubes spaced inwardly ofthe opposite end portions, assembling a number of such tubes with theiruncoated end portions nested together in surface contact, brazing oneface of the tube assembly by immersion of the nested ends in a heatconcentrating liquid to melt and flow the brazing material downwardlybetween the nested ends, and then turning the assembly end for end andsimilarly brazing the other face thereof. v LEROY W. SHUTIS. PHILIP S.ROSEN'BERG.

